The present invention relates to a femtosecond laser ophthalmological apparatus and method that creates a flap on the cornea for LASIK refractive surgery or for other applications that require removal of corneal and lens tissue at specific locations such as in corneal transplants, stromal tunnels, corneal lenticular extraction and cataract surgery.
The use of an excimer laser to modify the shape of the cornea is called Laser Vision Correction (LVC). Currently the most popular method is called LASIK (Laser-Assisted in-situ Keratomileusis) and accounts for approximately 85% of all LVC preformed. Traditionally, during LASIK, the surgeon uses an instrument called a mechanical microkeratome (physical blade) to create a flap on the cornea. However, over the last few years, femtosecond laser has increasingly been used to create a LASIK flap using a series of hundreds of thousands of small laser pulses to create a cleavage plane (“cut”) in the cornea.
Femtosecond laser created corneal flaps can offer greater safety, reproducibility, predictability and flexibility over mechanical microkeratome. Furthermore, complications such as buttonhole flaps (in very steep corneas), free caps (in very flat corneas) and irregular flaps that are associated with the mechanical microkeratome are rare with the femtosecond laser. Finally, femtosecond laser systems offer a wide range of other optical-related applications to include corneal transplants, stromal tunnels, corneal lenticular extraction and cataract surgery.
However, several limitations are associated with current femtosecond laser systems:
The overall size of current femtosecond laser systems are much larger than mechanical microkeratome systems. Concurrently, with the exception to Ziemer Ophthalmic AG's Femto LDV systems, current femtosecond laser systems require the patient's eye to be aligned to a fixed laser beam delivery point. These two factors negatively impacts patient and surgeon comfort during surgery. Whereas the corneal flap creation by mechanical microkeratome and subsequent corneal reshaping by an excimer laser system can be done without moving the patient, the size of femtosecond laser systems and fixed delivery require patients to be transferred from one location to another. It is not uncommon for patients to have to move to a separate room to receive corneal reshaping. The femtosecond laser created flaps also increases the surgery time (decreased workflow) as there is often a necessary wait time after laser flap creation (for cavitation gas bubbles to diffuse) before the patient can be moved. This is also a significant reason why most ophthalmology clinics in the world still employ mechanical microkeratome for more efficient workflow.
U.S. Pat. No. 7,621,637 by Rathjen describes an ophthalmological apparatus that Ziemer Ophthalmic AG's Femto LDV series currently utilizes, and it addresses the size, flexibility of delivery and surgery time (small laser spot size for smaller cavitation gas bubbles) issues previously mentioned. Rathjen proposes guiding the laser through a mirror-lens relay arm into a hand piece and attaching a suction unit at the end of the hand piece with a vacuum pump to secure the hand piece on the patient's eye. The system uses a line scanning pattern method of ablation. The line pattern uses a faster scanner to create a pattern and moves the pattern using a slower scanner to cover the necessary area of ablation. The pattern can be moved in a variety of ways to include rotation about a central axis to ablate the necessary area. This method of laser ablation is in contrast to the traditional method used by fixed delivery systems. The fixed delivery systems are able to map the area of the cornea out and have enough laser power and scanning speed to ablate the necessary pattern using a laser spot. This method is not available to a system that needs to guide the laser through an optical unit (mirror-lens relay arm) into a mobile hand piece. There are several disadvantages to Rathjen's method of approach. First, to avoid pattern distortion on the eye created by the translation motor during rotation of the line, the laser pulse line scanning pattern has to be precisely aligned to be perpendicular with the trajectory of the translation motor after passing through the delivery arm. Rathjen compensates with a rotation element to maintain the perpendicular trajectory. This creates a more complex, less reliable and potentially more expensive apparatus. Second, any pattern created by lines will have an inherent width (at a minimum the length of the line) that limits the flexibility of 3D trajectories available. Third, centering the suction ring on the eye while it is attached to the hand piece is cumbersome for surgeons. It requires additional fine movements to align laser spot center and eye center after the suction ring is connected to the eye. Finally, the current design uses a mirror-lens relay optical arm to deliver the laser beam from the main cabinet into the hand piece. Placing lenses in an optical arm amplifies alignment errors and creates a more complex module.
In the present invention an ophthalmological apparatus utilizes a femtosecond laser beam that travels through a rotating mirror set module as opposed to a mirror-lens relay optical arm. Using only mirrors simplifies the optical system's design and operation. The rotating mirror set module is attached to the main cabinet and a hand piece where the laser beam is deflected by a two dimensional XY scanner device into a predetermined pattern of laser pulses. The ablation pattern required is determined by dividing the ablation area into a matrix grid around 12×12 mm centered on the cornea of the eye. The matrix grid is further divided into individual cells and ablation is completed in each individual cell in a predetermined sequence until all cells in the matrix grid have been ablated. This method eliminates the need for compensating optics, rotation elements or trajectory limitations as encountered in the prior art. The suction ring is designed to be aligned and attached to eye center separate from the hand piece. Once the suction ring is fixed on the eye, the hand piece is moved to connect with the suction ring via a slide lock mechanism. This method ensures the suction ring and hand piece are both properly aligned.